Hydraulically powered machines employing repeating work cycles are common in manufacturing and heavy industry. Within the work cycles of such machines, it is common for the power demand to vary dramatically. Such power variations can present difficulties for designing efficient hydraulic drive systems. Low cost, energy efficient solutions are needed in this area.
FIG. 1 shows a prior art hydraulic system 18 including a hydraulic drive circuit 20 for powering operation of a machine 19 having a repeating work cycle (e.g., an injection molding machine). The machine 19 includes actuators 22a, 22b, 22c that are powered by the hydraulic drive circuit 20. The hydraulic drive circuit 20 includes a fixed displacement pump 24 driven by a constant speed electric motor 26. The pump 24 includes an inlet 28 and outlet 30. The inlet 28 connects to a reservoir 32 (i.e., a tank) and the outlet 30 connects to a relief valve 34 and flow control valve 36. The relief valve 34 controls the pump outlet pressure (i.e., the hydraulic system pressure) by passing extra flow to the reservoir 32. The flow control valve 36 controls the flow rate of the hydraulic fluid provided to the actuators. Valves 38a, 38b and 38c are used to selectively enable and disable the actuators 22a, 22b and 22c at different phases of the work cycle of the machine. During operation of the system, the hydraulic system pressure is controlled via the relief valve 34 to track the load pressure. The hydraulic system pressure typically exceeds the load pressure by a pressure margin that corresponds generally to a pressure drop across the flow control valve 36.
FIG. 2 depicts a system pressure profile 42 and a load pressure profile 44 for the hydraulic system 18. The system pressure profile 42 represents the hydraulic pressure at the pump outlet over the work cycle. The load pressure profile 44 represents the hydraulic pressure demand required by the load over the work cycle. As shown at FIG. 2, the system pressure profile 42 and the load pressure profile 44 track one another over the entire work cycle. The system pressure profile 42 and the load pressure profile 44 are separated by a margin 46 that corresponds to a pressure drop across the flow control valve 36. The system pressure is higher than the load pressure throughout the work cycle.
FIG. 3 depicts a system flow rate profile 48 and a flow demand profile 50 for the hydraulic system 18. The system flow rate profile 48 represents the hydraulic fluid output from the pump 24 over the work cycle. The flow demand profile 50 represents the flow required by the load over the work cycle. Since the pump is a fixed displacement pump driven by a constant speed motor, the system flow profile 48 is horizontal, thereby representing a constant flow output by the pump 24 over the duration of the work cycle. The electric motor 26 and the pump 24 need to be sized to meet peak power and peak flow demands. Therefore, a significant portion of the flow output from the pump 24 is passed to the reservoir 32 through the relief valve 34 without doing any useful work over the course of the full work cycle. If the peak power accounts for a small percentage of the overall work cycle, then a significant amount of energy is unused.
FIG. 4 illustrates hydraulic system 118 that uses another type of prior art hydraulic drive circuit 120 to drive an industrial machine having a repeating work cycle. The hydraulic drive circuit 120 has the same basic configuration as the hydraulic drive circuit 20 of FIG. 1, except for the addition of a hydraulic accumulator 60 and one-way check valve 62. The hydraulic accumulator 60 is connected between the pump 24 and the flow control valve 36. The one-way check valve 62 is installed between the accumulator 60 and the pump 24. The one-way check valve 62 prevents back-flow from the accumulator 60 toward the pump 24. Incorporating the accumulator 60 at the outlet side of the pump 24 filters pressure ripple and allows the pump 24 to be downsized. By including the accumulator 60, the pump 24 can be sized to provide the average flow required by the load over the work cycle.
FIG. 5 depicts a load pressure profile 64, an accumulator pressure profile 66 and a system pressure profile 68 for the work cycle of the hydraulic system 118. As shown in FIG. 5, the pump pressure (i.e., the system pressure) is maintained above the accumulator pressure throughout the entire work cycle. Also, the accumulator and pump pressure track one another throughout the work cycle
Referring to FIG. 6, a pump flow profile 70, an accumulator profile 72, a load flow profile 74 and a total flow profile 76 are depicted for the work cycle of the hydraulic system 118. The total flow profile 76 depicts the total flow provided by the combination of the pump 24 and the hydraulic accumulator 60. Since the pump 24 is a fixed displacement pump powered by a constant speed electric motor 26, the flow is constant thereby causing the pump flow profile 70 to be a horizontal line. The total flow profile 76 and the load profile 74 track one another. When the load flow demand is less than the pump flow, excess flow from the pump can be used to charge the accumulator 60. In contrast, when the load flow demand is greater than the pump flow, hydraulic fluid is discharged from the accumulator 60 so that the combined flow of the accumulator 60 and the pump 24 satisfies the load flow demand.
As shown at FIG. 6, the work cycle can be divided into alternating charge and discharge phases. For example, the work cycle has four charge phases (C1, C2, C3 and C4) and four discharge phases (D1, D2, D3 and D4). The system pressure is maintained higher than the accumulator pressure throughout the work cycle. Also, since the accumulator is positioned on the upstream side of the proportional valve, the flow from the accumulator has to pass through the proportional valve to reach the load. Under conditions where the load pressure is substantially lower than the system and accumulator pressures, significant throttling loss can be generated across the flow control valve 36.